The present invention relates to a hydraulic brake system comprising a master brake cylinder, wheel brake cylinders, a brake slip or antiskid control apparatus, a valve device controlling a fluid flow from a fluid source to a pressure chamber of the master cylinder, and valves actuated by the brake slip control apparatus controlling in the event of an imminent locked condition of a wheel the pressure in the associated wheel cylinder independently of the pressure chamber, the valve device being opened dependent on a differential of pressure between the pressure in the pressure chamber and the pressure in the wheel cylinder associated with the pressure chamber, the pressure in the wheel cylinder being required to be lower than the pressure in the pressure chamber by a predetermined amount.
In a copending U.S. patent application of J. Belart, Ser. No. 193,215, filed Oct. 2, 1980, disclosed, which provides for locking of the hydraulic booster piston only if the change-over pressure at which the static system is switched to the dynamic system is attained. Below this change-over point the booster piston will not be locked in position with the brake slip control apparatus in operation, so that even with additional fluid supply from the dynamic circuit into the static circuit, movement of the brake pedal through its full travel cannot be prevented.